This invention relates to an organic electroluminescent (EL) device and more particularly, to a device capable of emitting light from a thin film of an organic compound upon application of electric field.
Organic EL devices are light emitting devices comprising a thin film containing a fluorescent organic compound interleaved between a cathode and an anode. Electrons and holes are injected into the thin film where they are recombined to create excitons. Light is emitted by utilizing luminescence (phosphorescence or fluorescence) upon deactivation of excitons.
The organic EL devices are characterized by plane light emission at a high luminance of about 100 to 100,000 cd/m2 with a low voltage of about 10 volts and light emission in a spectrum from blue to red color by a simple choice of the type of fluorescent material.
The organic EL devices, however, are undesirably short in emission life, less durable on storage and less reliable because of the following factors.
(1) Physical Changes of Organic Compounds
Growth of crystal domains renders the interface non-uniform, which causes deterioration of electric charge injection ability, short-circuiting and dielectric breakdown of the device. Particularly when a low molecular weight compound having a molecular weight of less than 500 is used, crystal grains develop and grow, substantially detracting from film quality. Even when the interface with ITO is rough, significant development and growth of crystal grains occur to lower luminous efficiency and allow current leakage, ceasing to emit light. Dark spots which are local non-emitting areas are also formed.
(2) Oxidation and Stripping of the Cathode
Although metals having a low work function such as Na, Mg, Li, Ca, K, and Al are used as the cathode in order to facilitate electron injection, these metals are reactive with oxygen and moisture in air. As a result, the cathode can be stripped from the organic compound layer, prohibiting electric charge injection. Particularly when a polymer or the like is applied as by spin coating, the residual solvent and decomposed products resulting from film formation promote oxidative reaction of the electrodes which can be stripped to create local dark spots.
(3) Low Luminous Efficiency and Increased Heat Build-up
Since electric current is conducted across an organic compound, the organic compound must be placed under an electric field of high strength and cannot help heating. The heat causes melting, crystallization or decomposition of the organic compound, leading to deterioration or failure of the device.
(4) Photochemical and Electrochemical Changes of Organic Compound Layers
Coumarin compounds were proposed as the fluorescent material for organic EL devices (see JP-A 264692/1988, 191694/1990, 792/1991, 202356/1993, 9952/1994, and 240243/1994). The coumarin compounds are used in the light emitting layer alone or as a guest compound or dopant in admixture with host compounds such as tris(8-quinolinolato)-aluminum. Such organic EL devices have combined with the light emitting layer a hole injecting layer, a hole transporting layer or a hole injecting and transporting layer which uses tetraphenyldiamine derivatives based on a 1,1xe2x80x2-biphenyl-4,4xe2x80x2-diamine skeleton and having phenyl or substituted phenyl groups attached to the two nitrogen atoms of the diamine, for example, N,Nxe2x80x2-diphenyl-N,Nxe2x80x2-bis(3-methylphenyl)-1,1xe2x80x2-biphenyl-4,4xe2x80x2-diamine. These organic EL devices, however, are unsatisfactory in emission life and reliability with respect to heat resistance. When these compounds are used as a host, high luminance devices are not available.
To meet the demand for organic EL devices of the multi-color light emission type, multilayer white light emitting organic EL devices were proposed (Yoshiharu Sato, Shingaku Giho, OME94-78 (1995-03)). The light emitting layer used therein is a lamination of a blue light emitting layer using a zinc oxazole complex, a green light emitting layer using tris(8-quinolinolato)aluminum, and a red light emitting layer of tris(8-quinolinolato)aluminum doped with a red fluorescent dye (P-660, DCM1).
The red light emitting layer is doped with a luminescent species to enable red light emission as mentioned above while the other layers are subject to no doping. For the green and blue light emitting layers, a choice is made such that light emission is possible with host materials alone. The choice of material and the freedom of adjustment of emission color are severely constrained.
In general, the emission color of an organic EL device is changed by adding a trace amount of a luminescent species, that is, doping. This is due to the advantage that the luminescent species can be readily changed by changing the type of dopant. Accordingly, multi-color light emission is possible in principle by doping a plurality of luminescent species. If a single host is evenly doped with all such luminescent species, however, only one of the luminescent species doped would contribute to light emission or some of the luminescent species dopes would not contribute to light emission. In summary, even when a single host is doped with a mixture of dopants, it is difficult for all the dopants to contribute to light emission. This is because of the tendency that energy is transferred to only a particular luminescent species.
For this and other reasons, there are known until now no examples of doping two or more luminescent species so that stable light emission may be derived from them.
In general, the luminance half-life of organic EL devices is in a trade-off to the luminescence intensity. It was reported (Tetsuo Tsutsui, Applied Physics, vol. 66, No. 2 (1997)) that the life can be prolonged by doping tris(8-quinolinolato)aluminum or N,Nxe2x80x2-diphenyl-N,Nxe2x80x2-bis(3-methylphenyl)-1,1xe2x80x2-biphenyl-4,4xe2x80x2-diamine with rubrene. A device having an initial luminance of about 500 cd/m2 and a luminance half-life of about 3,500 hours was available. The emission color of this device is, however, limited to yellow (in proximity to 560 nm). A longer life is desired.
An object of the present invention is to provide an organic EL device using a photoelectric functional material experiencing minimal physical changes, photochemical changes or electrochemical changes and capable of light emission of plural colors at a high luminous efficiency in a highly reliable manner. Another object is especially to provide a high luminance light emitting device using an organic thin film formed from a high molecular weight compound by evaporation, the device being highly reliable in that a rise of drive voltage, a drop of luminance, current leakage, and the appearance and development of local dark spots during operation of the device are restrained. A further object is to provide an organic EL device adapted for multi-color light emission and capable of adjustment of an emission spectrum. A still further object is to provide an organic EL device featuring a high luminance and a long lifetime.
These and other objects are attained by the present invention which is defined below as (1) to (18).
(1) An organic electroluminescent device comprising
a light emitting layer containing a coumarin derivative of the following formula (I), and
a hole injecting and/or transporting layer containing a tetraaryldiamine derivative of the following formula (II), 
xe2x80x83wherein each of R1, R2, and R3, which may be identical or different, is a hydrogen atom, cyano, carboxyl, alkyl, aryl, acyl, ester or heterocyclic group, or R1 to R3, taken together, may form a ring; each of R4 and R7 is a hydrogen atom, alkyl or aryl group; each of R5 and R6 is an alkyl or aryl group; or R4 and R5, R5 and R6, and R6 and R7, taken together, may form a ring, and 
xe2x80x83wherein each of Ar1, Ar2, Ar3, and Ar4 is an aryl group, at least one of Ar1 to Ar4 is a polycyclic aryl group derived from a fused ring or ring cluster having at least two benzene rings; each of R11 and R12 is an alkyl group; each of p and q is 0 or an integer of 1 to 4; each of R13 and R14 is an aryl group; and each of r and s is 0 or an integer of 1 to 5.
(2) The organic electroluminescent device of (1) wherein said light emitting layer containing a coumarin derivative is formed of a host material doped with the coumarin derivative as a dopant.
(3) The organic electroluminescent device of (2) wherein said host material is a quinolinolato metal complex.
(4) An organic electroluminescent device comprising a light emitting layer in the form of a mix layer containing a hole injecting and transporting compound and an electron injecting and transporting compound, the mix layer being further doped with a coumarin derivative of the following formula (I), a quinacridone compound of the following formula (III) or a styryl amine compound of the following formula (IV) as a dopant, 
xe2x80x83wherein each of R1, R2, and R3, which may be identical or different, is a hydrogen atom, cyano, carboxyl, alkyl, aryl, acyl, ester or heterocyclic group, or R1 to R3, taken together, may form a ring; each of R4 and R7 is a hydrogen atom, alkyl or aryl group; each of R5 and R6 is an alkyl or aryl group; or R4 and R5, R5 and R6, and R6 and R7, taken together, may form a ring, 
xe2x80x83wherein each of R21 and R22, which may be identical or different, is a hydrogen atom, alkyl or aryl group; each of R23 and R24 is an alkyl or aryl group; each of t and u is 0 or an integer of 1 to 4; or adjacent R23 groups or R24 groups, taken together, may form a ring when t or u is at least 2, 
xe2x80x83wherein R31 is a hydrogen atom or aryl group; each of R32 and R33, which may be identical or different, is a hydrogen atom, aryl or alkenyl group; R34 is an arylamino or arylaminoaryl group; and v is 0 or an integer of 1 to 5.
(5) The organic electroluminescent device of (4) wherein said hole injecting and transporting compound is an aromatic tertiary amine, and said electron injecting and transporting compound is a quinolinolato metal complex.
(6) The organic electroluminescent device of (5) wherein said aromatic tertiary amine is a tetraaryldiamine derivative of the following formula (II): 
xe2x80x83wherein each of Ar1, Ar2, Ar3, and Ar4 is an aryl group, at least one of Ar1 to Ar4 is a polycyclic aryl group derived from a fused ring or ring cluster having at least two benzene rings; each of R11 and R12 is an alkyl group; each of p and q is 0 or an integer of 1 to 4; each of R13 and R14 is an aryl group; and each of r and s is 0 or an integer of 1 to 5.
(7) The organic electroluminescent device of any one of (1) to (6) wherein said light emitting layer is interleaved between at least one hole injecting and/or transporting layer and at least one electron injecting and/or transporting layer.
(8) The organic electroluminescent device of (1), (2), (3) or (7) wherein said hole injecting and/or transporting layer is further doped with a rubrene as a dopant.
(9) The organic electroluminescent device of any one of (1) to (8) wherein a color filter and/or a fluorescence conversion filter is disposed on a light output side so that light is emitted through the color filter and/or fluorescence conversion filter.
(10) An organic electroluminescent device comprising at least two light emitting layers including a bipolar light emitting layer, a hole injecting and/or transporting layer disposed nearer to an anode than said light emitting layer, and an electron injecting and/or transporting layer disposed nearer to a cathode than said light emitting layer,
said at least two light emitting layers being a combination of bipolar light emitting layers or a combination of a bipolar light emitting layer with a hole transporting/light emitting layer disposed nearer to the anode than the bipolar light emitting layer and/or an electron transporting/light emitting layer disposed nearer to the cathode than the bipolar light emitting layer.
(11) The organic electroluminescent device of (10) wherein said bipolar light emitting layer is a mix layer containing a hole injecting and transporting compound and an electron injecting and transporting compound.
(12) The organic electroluminescent device of (11) wherein all said at least two light emitting layers are mix layers as defined above.
(13) The organic electroluminescent device of any one of (10) to (12) wherein at least one of said at least two light emitting layers is doped with a dopant.
(14) The organic electroluminescent device of any one of (10) to (13) wherein all said at least two light emitting layers are doped with dopants.
(15) The organic electroluminescent device of any one of (10) to (14) wherein said at least two light emitting layers have different luminescent characteristics, a light emitting layer having an emission maximum wavelength on a longer wavelength side is disposed near the anode.
(16) The organic electroluminescent device of any one of (13) to (15) wherein said dopant is a compound having a naphthacene skeleton.
(17) The organic electroluminescent device of any one of (13) to (16) wherein said dopant is a coumarin of the following formula (I): 
xe2x80x83wherein each of R1, R2, and R3, which may be identical or different, is a hydrogen atom, cyano, carboxyl, alkyl, aryl, acyl, ester or heterocyclic group, or R1 to R3, taken together, may form a ring; each of R4 and R7 is a hydrogen atom, alkyl or aryl group; each of R5 and R6 is an alkyl or aryl group; or R4 and R5, R5 and R6, and R6 and R7, taken together, may form a ring.
(18) The organic electroluminescent device of any one of (11) to (17) wherein said hole injecting and transporting compound is an aromatic tertiary amine, and said electron injecting and transporting compound is a quinolinolato metal complex.
The organic EL device of the invention can achieve a high luminance of about 100,000 cd/m2 or higher in a stable manner since it uses a coumarin derivative of formula (I) in a light emitting layer and a tetraaryldiamine derivative of formula (II) in a hole injecting and/or transporting layer, or a light emitting layer is formed by doping a mix layer of a hole injecting and transporting compound and an electron injecting and transporting compound with a coumarin derivative of formula (I), a quinacridone compound of formula (II) or a styryl amine compound of formula (III). A choice of a highly durable host material for the coumarin derivative of formula (I) allows for stable driving of the device for a prolonged period even at a current density of about 30 mA/cm2.
Since evaporated films of the above-mentioned compounds are all in a stable amorphous state, thin film properties are good enough to enable uniform light emission free of local variations. The films remain stable and undergo no crystallization over one year in the air.
Also the organic EL device of the invention is capable of efficient light emission under low drive voltage and low drive current conditions. The organic EL device of the invention has a maximum wavelength of light emission in the range of about 480 nm to about 640 nm. For example, JP-A 240243/1994 discloses an organic EL device comprising a light emitting layer using tris(8-quinolinolato)aluminum as a host material and a compound embraced within the coumarin derivatives of formula (I) according to the present invention as a guest material. However, the compound used in the hole transporting layer is N,Nxe2x80x2-diphenyl-N,Nxe2x80x2-bis(3-methylphenyl)-1,1xe2x80x2-biphenyl-4,4xe2x80x2-diamine and thus different from the compounds of formula (II) according to the present invention. There are known no examples of doping a light emitting layer of the mix layer type with a coumarin a derivative of formula (I), a quinacridone compound of formula (II) or a styryl amine compound of formula (III).
Furthermore, in order to enable light emission of two or more colors by altering the carrier transporting capability of respective light emitting layers, the present invention employs two or more light emitting layers, at least one of which is a layer of the bipolar type, preferably of the mix layer type, and which are a combination of bipolar light emitting layers, preferably of the mix layer type or a combination of a bipolar light emitting layer, preferably of the mix layer type with a hole transporting/light emitting layer disposed nearer to the anode than the bipolar light emitting layer, preferably of the mix layer type and/or an electron transporting/light emitting layer disposed nearer to the cathode than the bipolar light emitting layer. Further preferably, the light emitting layers are doped with respective dopants.
Among the foregoing embodiments, the especially preferred embodiment wherein a mix layer is doped is discussed below. By providing a mix layer and doping it, the recombination region is spread throughout the mix layer and to the vicinity of the interface between the mix layer and the hole transporting/light emitting layer or the interface between the mix layer and the electron transporting/light emitting layer to create excitons whereupon energy is transferred from the hosts of the respective light emitting layers to the nearest luminescent species to enable light emission of two or more luminescent species (or dopants). Also in the embodiment using the mix layer, by selecting for the mix layer a compound which is stable to the injection of holes and electrons, the electron and hole resistance of the mix layer itself can be outstandingly improved. In contrast, a combination of a hole transporting/light emitting layer with an electron transporting/light emitting layer rather in the absence of a mix layer which is a bipolar light emitting layer enables light emission from two or more luminescent species, but is so difficult to control the light emitting layers that the ratio of two luminescence intensities will readily change, and is short in life and practically unacceptable because these light emitting layers are less resistant to both holes and electrons. Also it becomes possible to adjust the carrier (electron and hole) providing capability by adjusting the combination of host materials for light emitting layers, the combination and quantity ratio of host materials for mix layers which are bipolar light emitting layers, or the ratio of film thicknesses. This enables adjustment of a light emission spectrum. The present invention is thus applicable to an organic EL device of the multi-color light emission type. In the embodiment wherein a light emitting layer (especially a mix layer) doped with a naphthacene skeleton bearing compound such as rubrene is provided, owing to the function of the rubrene-doped layer as a carrier trapping layer, the carrier injection into an adjacent layer (e.g., an electron transporting layer or a hole transporting layer) is reduced to prohibit deterioration of these layers, leading to a high luminance of about 1,000 cd/m2 and a long lifetime as expressed by a luminance half-life of about 50,000 hours. In the further embodiment wherein a light emitting layer having a maximum wavelength of light emission on a longer wavelength side is disposed near the anode, a higher luminance is achievable because the optical interference effect can be utilized and the efficiency of taking out emission from the respective layers is improved.
Although an organic EL device capable of white light emission is proposed in Shingaku Giho, OME94-78 (1995-03), no reference is made therein to the doping of two or more light emitting layers including a bipolar light emitting layer, especially a mix layer as in the present invention.